Several procedures for the oxidation and/or reduction of water to yield hydrogen and oxygen by the aid of metallic catalysts have been disclosed. The catalysts employed so far for this latter purpose are lanthanide-type photocatalysts, such as NaTaO3:La, catalysts based on rare earth metals, such as R2Ti2O7 (R=Y, rare earth), or TiO2-derived semiconductor materials arranged in a so-called tandem cell. Notably, in these procedures no mention of the use of silicides and silicide-like compositions and/or oxides thereof has been made for the title applications.
The processes for the generation of hydrogen and oxygen from water comprise reduction and/or oxidation processes using semiconductors and light. These processes are also called in summa water splitting processes. The hitherto disclosed procedures employ UV light. Although in some cases remarkable amounts of hydrogen and oxygen evolution is observed, the irradiation conditions are not suitable for solar applications. Further, the preparations of the catalysts are laborious and require uneconomically high temperatures, starting from expensive materials of very high purity. Furthermore, these processes require water of very high purity, i.e. tri-distilled water. Of the cases no indication concerning longer time applications including the consequences for the stability of the catalysts is made.
Therefore subject matter of present invention is a process for the photo- and thermochemical production/generation of hydrogen and/or oxygen wherein water is brought into contact with suicides and silicide-like compositions and/or oxides thereof., i.e. compositions all containing silicon and oxides thereof and being of the molecular formula RSixOy wherein R represents pure or mixed organic, metallic, organometallic and/or biochemically derived residues and/or inorganic residues, and Si being silicon and specifically a silicide moiety with X>0 and O is oxygen with Y 0. The silicide moieties in these compositions exhibit characteristically a high electron density at silicon. The silicides and silicide-like compositions and/or oxides thereof can react catalytically in these aforementioned processes proceeding with or without light. However, upon irradiation of the reactions an increase of gas evolution is observed, this notably applying to artificial light as well as sunlight. Higher reaction temperatures are often favourable for these processes. Suicides and silicide-like compositions and/or oxides thereof are mostly semiconductor-type materials. Furthermore, these compositions are able to store/release and/or absorb/desorb hydrogen and oxygen reversibly wherein oxygen storage/release and/or absorption/desorption is favourable but can occur simultaneously with hydrogen storage/absorption and desorption/release. The release/desorption of hydrogen and oxygen can occur at ambient or higher temperatures, especially the processes concerning hydrogen, depending on the nature of the suicides and silicide-like compositions and/or oxides thereof employed.
Furthermore, it was found that coupling/complexing/attaching/binding of a dye such as perylenes, perylene dyes and perylene congeneers/analogs to silicides and silicide-like compositions and/or oxides thereof is favourable for the light absorption and hence reactivity of the silicides and silicide-like compositions and/or oxides thereof.
Further, it was found that the reactions using suicides and silicide-like compositions and/or oxides thereof for the purpose of water reduction and/or oxidation to yield hydrogen and/or oxygen, respectively, can be carried out by employing the silicides and silicide-like compositions and/or oxides thereof in immobilized form, i.e. when these compositions are attached/fixed onto or in a polymeric surface or material, as well as onto or in a glass or glass-like material, especially when the polymeric and/or glass-type material is electrically conducting.
Further, it was found that the storage/release and/or absorption/desorption of hydrogen and/or oxygen using silicides and silicide-like compositions and/or oxides thereof when these compositions are immobilized, i.e. when these materials are attached/fixed onto or in a polymeric surface and/or glass and/or glass-like material, this in processes carried out with or without light.
Furthermore, processes wherein oxygen is transformed to polyoxygen of the formula On (n>3) and/or hydogenpolyperoxides of the formula H2On (n>2) including the back reactions to form oxygen again have not been described in literature so far, but have been found experimentally here; theoretical studies based on calculation predicting a shallow energy minimum and hence low to questionable stability for polyoxygen and hydrogenpolyperoxides in the gas phase. However, the experienced stability of polyoxygen and hydrogenpolyperoxides is seemingly due to stabilization in solution and/or by a metal.